Compare it with a car ...
You can ask me to prepare your car so it's good to race with.
BUT I can not give any guarantee that you will win the race. Understand? For example you can always fuck up yourself.
If you want me to guarantee that you win the race , then I ask 95% of the profit you make from it (lifetime).
Why? Because then I WON YOUR RACE, NOT YOU. Capish?
In the first place SEO companies make that your site is all set and everything is in place to make that you can "join the race for some good portion organic traffic".
There is not a single SEO company in the world who will guarantee certain results.
But I can assure you that they'll do all possible, because when you are a happy costumer then new clients will step inside.
Read this:
https://moz.com/blog/why-reputable-s...ngine-rankings